The invention relates to a method for operating an internal combustion engine.
For operating internal combustion engines, the ratio of fuel and air must be controlled such that the mixture in the combustion chamber can be safely ignited. In internal combustion engines of motor vehicles usually a lambda probe is arranged in the exhaust gas stream. In small internal combustion engines of a simple configuration as they are used, for example, in hand-held power tools such as motor chainsaws, trimmers, lawnmowers or the like a lambda probe is usually not provided. The control of the mixture composition is realized in these engines in that a disruption in the form of a change of the supplied fuel quantity is introduced and the resulting engine speed reaction of the engine is evaluated. Depending on the operating point of the internal combustion engine, after a brief reduction or increase of the injected fuel quantity an increase of the engine speed or a drop of the engine speed occurs. This engine speed reaction can be detected and based thereon the operating point of the internal combustion engine can be derived. Based on the determined operating point, the desired operating point can then be adjusted.
The change of engine speed is very slow, in particular when introducing fuel into the intake passage or into the crankcase of a two-stroke engine. Therefore, it takes a few revolutions of the crankshaft until an engine speed reaction can be determined. When the mixture in the combustion chamber is very lean and the supplied fuel quantity is further reduced for determining the operating point, this can lead to the engine stalling. In case of very lean mixtures there is moreover the risk of motor damage especially as a result of unsatisfactory lubrication.